U.S. patent application Ser. No. 07/795,034, filed Nov. 20, 1991 (now U.S. Pat. No. 5,227,498), U.S. patent application Ser. No. 07/979,250, filed Nov. 20, 1992 and International Application PCT/US92/09992 (Publication No. WO 93/09956, all describe amino-substituted squarylium infra-red dyes, including (in the last two applications) dyes containing 6-alkoxybenzpyrylium nuclei.
U.S. patent application Ser. No. 07/795,038, filed Nov. 20, 1991, describes and claims bis(benzpyrylium) squarylium dyes, including certain asymmetric dyes produced by processes of the present invention.
U.S. patent application Ser. No. 07/965,161, filed Oct 23, 1992 now U.S. Pat. No. 5,286,612, describes a process for generation of acid, which process comprises:
providing a medium containing a mixture of a superacid precursor and a dye capable of absorbing actinic radiation of a first wavelength which does not, in the absence of the dye, cause decomposition of the superacid precursor to form the corresponding superacid, the superacid precursor being capable of being decomposed by actinic radiation of a second wavelength shorter than the first wavelength;
irradiating the medium with actinic radiation of the first wavelength, thereby causing absorption of the actinic radiation, and decomposition of part of the superacid precursor, without formation of free superacid but with formation of a protonated product derived from the dye; and
thereafter irradiating the medium with actinic radiation of the second wavelength, thereby causing decomposition of part of the remaining superacid precursor, with formation of free superacid.
At least some of the dyes produced by the process of the present invention may be used in the process of this copending application.
The disclosures of the aforementioned U.S. applications and patents are herein incorporated by reference.
This invention relates to squarylium compounds, and processes and intermediates for the synthesis of these compounds. More specifically, this invention relates to processes and intermediates useful for the synthesis of squarate dyes (and to such dyes themselves) in which two heterocyclic nuclei are linked to the 1- and 3-positions of a squarate ring via a single sp2 hybridized carbon atom (hereinafter called the xe2x80x9cmesoxe2x80x9d carbon atom); these dyes will hereinafter be called xe2x80x9cpentamethine squarate dyesxe2x80x9d. The processes of the present invention are especially useful for the synthesis of asymmetric pentamethine squarate dyes, i.e., those in which the two heterocyclic nuclei are dissimilar. The present invention is also useful for the synthesis of related dyes in which one meso carbon atom and its associated heterocyclic nucleus are replaced by an aromatic nucleus directly bonded to the squarylium ring.
It is known that compounds in which two heterocyclic nuclei are linked by a pentamethine chain, the three central carbon atoms of which form part of a squarate ring, are useful as dyes, especially near infra-red dyes. (The term xe2x80x9cnear infra-redxe2x80x9d is used herein to mean electromagnetic radiation having a wavelength of about 700 to about 1200 nm.) For example, Japanese Patent Application No. 103,604/82 (Publication No. 220,143/83, published Dec. 21, 1983), discloses a broad class of bis-heterocyclic pentamethine dyes in which the central three carbon atoms of the pentamethine chain form part of a squarylium or croconylium ring. The heterocyclic nuclei can be pyrylium, thiopyrylium, selenopyrylium, benzpyrylium, benzthiopyrylium, benzselenopyrylium, naphthopyrylium, naphthothiopyrylium or naphthoselenopyrylium nuclei, which can be substituted with alkyl, alkoxy, aryl or styryl groups.
Japanese Patent Application No. 60-8730 (Publication No. 167,681/86, published Jul. 29, 1986), discloses bis(4-benz[b]thiopyrylium) pentamethine dyes in which the central three carbon atoms of the pentamethine chain form part of a squarylium ring. The dyes are intended for use as infra-red absorbers.
U.S. Pat. No. 4,508,811, issued Apr. 2, 1985, describes an optical recording element in which the recording layer comprises a bis(2,6-dialkyl)-pyrylium or -thiopyrylium squarylium salt.
Application Ser. No. 07/616,639, filed Nov. 21, 1990 (now abandoned) by Stephen J. Telfer et al. and assigned to the same assignee as the present application, and the aforementioned U.S. patent application Ser. No. 07/795,038, describe 4-[[3-[(benz[b]-4H-pyran-4-ylidene)methyl]-2-hydroxy-4-oxo-2-cyclobuten-1-ylidene]methyl]benz[b]pyrylium hydroxide inner salt dyes, in which at least one benzpyrylium nucleus carries at its 2-position a substituent in which a non-aromatic carbon atom is bonded directly to the benzpyrylium nucleus, subject to the proviso that if this 2-substituent contains an aromatic nucleus, this aromatic nucleus is not conjugated with the benzpyrylium nucleus. These dyes have high absorptions in the near infra-red, and improved solubility in semi-polar solvents and plastics.
Most of these aforementioned pentamethine squarate dyes are symmetrical, that is to say the two heterocyclic nuclei are the same. Such symmetrical dyes are typically prepared by condensing two moles of the appropriate alkyl-substituted heterocyclic compound (usually, a salt) with squaric acid in the presence of a-base.
In certain applications of pentamethine squarate dyes, it may be advantageous to use a dye which is asymmetric, i.e., which contains two different heterocyclic groupings. For example, some symmetrical near infra-red pentamethine squarate dyes have significant absorption in the visible region, and this visible absorption restricts the utility of the dyes in certain applications, for example thermal imaging media. In particular, if the symmetrical dye absorbs strongly in one part of the visible spectrum but not in another, it will tend to introduce color distortion into any image created using the symmetrical dye. Although asymmetrical analogues of these infra-red pentamethine squarate dyes may have some visible absorption, this visible absorption tends to take the form of several separate small peaks, and is thus more spread out over a wide range of wavelengths than in the symmetrical dyes. Such absorption over a range of wavelengths tends to produce lower peak absorption and less color distortion (because the dye tends to produce a grey tint) than that produced by the symmetrical dyes, and thus the asymmetric dyes may advantageously be used in applications where the visible absorption of the symmetric dyes causes problems.
Moreover, there are a number of applications where infra-red dyes are needed which absorb at specific wavelengths. For example U.S. Pat. Nos. 4,602,263 and 4,826,976 both describe thermal imaging systems for optical recording and particularly for forming color images. These patents describe a preferred form of thermal imaging medium for forming multicolor images; in this preferred imaging medium, three separate color-forming layers, capable of forming yellow, cyan and magenta dyes respectively, are superposed on top of one another. Each of the three color-forming layers has an associated infra-red absorber, these absorbers absorbing at differing wavelengths, for example 760, 820 and 880 nm. This medium is imagewise exposed simultaneously to three lasers having wavelengths of 760, 820 and 880 nm. The resultant imagewise heating of the color-forming layers causes the leuco dyes to undergo color changes in the exposed areas, thus producing a multicolored image, which needs no development. If the choice of infra-red dyes is restricted to symmetrical compounds, it may be difficult to find a dye which absorbs at the precise wavelength required, and which meets the other requirements, such as storage stability and miscibility in polymers, for use in such media. Asymmetric dyes, which allow the two groups linked to the squarylium nucleus to be varied independently, provide an extra degree of freedom, which renders it easier to find a dye which absorbs at the desired wavelength and meets the other requirements for use in such media.
However, despite the potential advantages of asymmetric pentamethine squarate dyes, little research has been conducted on such dyes because of the difficulties involved in their synthesis. Although it is possible to modify the conventional alkyl-substituted heterocyclic compound/squaric acid condensation reaction to produce asymmetric pentamethine dyes by including two different heterocyclic compounds in the reaction mixture, such a modified process inevitably produces three different products (two symmetrical dyes and the desired asymmetric dye), thus wasting at least half the starting materials (and possibly more if one heterocyclic compound is significantly more reactive than the other). Given that the costs of some symmetric pentamethine squarate dyes are high, such materials should be used judiciously and their loss minimized where possible.
Furthermore, separation of the tertiary product mixture produced is difficult, especially since, in many cases of practical importance, the two heterocyclic compounds used are chemically similar. For example, if one attempts to produce the dye of Formula A shown in FIG. 1 in which R1 and R2 are each a hydrogen atom (this dye contains one pyrylium nucleus and one selenopyrylium nucleus) simply by condensing a mixture of the two corresponding salts with squaric acid, it is extremely difficult to separate the desired asymmetric salt from the two, even on a laboratory scale, and conducting this separation on a commercial scale would be a practical impossibility. In some applications of infra-red dyes, the presence of even minor amounts of symmetric by-products in the desired asymmetric dye may cause significant problems. For example, as already noted, in the thermal imaging media described in the aforementioned U.S. Pat. Nos. 4,602,263 and 4,826,976, three separate imaging layers are present having infra-red absorbers with absorptions at 760, 820 and 880 nm. Conveniently, two of these three absorbers are Dye A shown in FIG. 1, in which R1 and R2 are each a hydrogen atom, and the corresponding bis-selenopyrylium dye. However, if Dye A is contaminated with even a small proportion of the corresponding bis-selenopyrylium dye, serious problems may result in such a medium, in that the bis-selenopyrylium impurity in the layer containing Dye A will absorb the xe2x80x9cwrongxe2x80x9d radiation, which may lead to unwanted exposure of parts of the layer containing Dye A and a reduction in sensitivity of the medium because the bis-selenopyrylium impurity will absorb a large part of the radiation intended to cause color change in a different color-forming layer.
There is thus a need for a process for the preparation of pentamethine squarate dyes, which does not require the separation of mixtures of asymmetric and symmetric products, and which can avoid waste of starting materials.
Processes for the preparation of asymmetric compounds, in which two different aromatic nuclei are directly bonded to a squarate ring, are known. Kazmaier et al., xe2x80x9cThe Photogenerating Properties of Unsymmetrical Squaraines and Squaraine Compositesxe2x80x9d, J. Imag. Sci., 32, 1-4 (1988) states that unsymmetrical squaraines can be produced by a two-step route in which the two pendent aromatic groups are attached in separate reactions, and further states that xe2x80x9cUnsymmetrical squaraines were synthesized in a multi-step procedure featuring the preparation of 4-(4-dimethylaminophenyl)-3-hydroxycyclobutenedionexe2x80x9d. However, no further details of this procedure are given.
U.S. Pat. No. 4,751,327 and U.S. Pat. No. 4,624,904 describe unsymmetrical squaraines for use in photoconductive imaging members. Columns 8-10 of each patent describe two synthetic methods for the preparation of these squaraines, these methods involving condensation of a diacid chloride or diester of squaric acid with one mole of a first amine, to form the appropriate 4-aminophenyl squarate derivative, hydrolysis of this derivative to introduce a 2-hydroxyl group on the squarate ring, and a second condensation to introduce at the 3-position of the squarate ring a second and different 4-aminophenyl group.
U.S. Pat. No. 4,922,018 and U.S. Pat. No. 4,886,722 describe unsymmetrical squaraines and their use in photoconductive imaging members. These squaraines are prepared by condensing, for example, a 1-alkoxyaryl-2-hydroxycyclobutene-3,4-dione derivative with an N,N-dialkylaniline derivative in the presence of an aliphatic alcohol and optionally a drying reagent. The squarate derivative is formed by a 2+2 cycloaddition process involving a tetraalkoxyolefin and an alkoxyarylketene generated in situ by the reaction of an alkoxyarylacetyl chloride and a base. The conditions of this cycloaddition reaction limit the substituents which can be present on the alkoxyarylacetyl chloride. Furthermore, the syntheses of the alkoxyarylacetyl chlorides required may be difficult.
The present invention provides processes, which can be used to prepare asymmetric pentamethine squarate and related dyes, and intermediates produced by such processes.
This invention provides a process for the preparation of a squarylium compound of the formula: 
wherein Q1 and Q2 are each independently an aromatic heterocyclic nucleus such that in the compounds of formulae Q1CH2R1 and Q2CH2R2 the methylene hydrogens are active hydrogens, and R1 and R2 are each independently a hydrogen atom or an aliphatic or cycloaliphatic group. This process comprises reacting a squaric acid derivative of the formula: 
wherein Q1 and R1 are as defined above, with a compound of the formula Q2CH2R2. This reaction, hereinafter called the xe2x80x9csalt dye-formingxe2x80x9d reaction of the invention, is desirably conducted in the presence of either a base or a Lewis acid (for example, titanium tetrachloride).
This invention also provides a squarylium compound of Formula I above wherein Q1 and Q2 are each independently a pyrylium, thiopyrylium, selenopyrylium, benzpyrylium, benzthiopyrylium or benzselenopyrylium nucleus, and R1 and R2 are each independently a hydrogen atom or an aliphatic or cycloaliphatic group, the Q1CR1 grouping being different from the Q2CR2 grouping.
This invention also provides a first process for the preparation of a squaric acid derivative of Formula II as defined above, which process comprises hydrolyzing a trihalosquaric acid derivative of the formula: 
wherein Q1 and R1 are as defined above, and X represents chlorine or bromine. This process will hereinafter be called the xe2x80x9ctrihalosquaric hydrolysisxe2x80x9d reaction of the invention.
This invention also provides a second process for the preparation of a squaric acid derivative of Formula II as defined above, which process comprises reacting a diester, monoacid chloride monoester or diacid chloride of squaric acid with a compound of the formula Q1CH2R1 (wherein Q1 is a heterocyclic nucleus such that in the compound of formula Q1CH2R1 the methylene hydrogens are active hydrogens, subject to the proviso that in Q1 a carbon atom is bonded to the carbon atom carrying the group R1, and this carbon atom is not bonded directly to a nitrogen atom, and R1 is as defined above), followed by hydrolysis of the resultant monoacid chloride or monoester intermediate. This reaction will hereinafter be called the xe2x80x9csalt condensationxe2x80x9d reaction of the invention.
This invention also provides a third process for the preparation of a squaric acid derivative of the formula: 
wherein Q1 is a 4-pyrylium, 4-thiopyrylium, 4-selenopyrylium, 4-benzpyrylium, 4-benzthiopyrylium or 4-benzselenopyrylium nucleus, R1 is a hydrogen atom or an aliphatic or cycloaliphatic group, and Axe2x80x2 is an esterified hydroxyl group; the compounds of Formula IV are of course esters of the compounds of Formula II.
This process comprises reacting a chromone of the formula Q1xe2x95x90O with a squaric acid derivative of the formula: 
This process will hereinafter be called the xe2x80x9cchromone condensationxe2x80x9d reaction of the invention. The resultant ester of Formula IV may of course be hydrolyzed to the corresponding hydroxyl compound of Formula II by conventional methods.
This invention also provides a process for the preparation of a trihalosquaric acid derivative of Formula (III) as defined above, which process comprises condensing a 2,3,4,4-tetrahalocyclobut-2-en-1-one with a compound of the formula Q1CH2R1 in the presence of a base. This reaction will hereinafter be called the xe2x80x9ctrihalosquaric formation reactionxe2x80x9d.
This invention also provides a squaric acid derivative of Formula III above, in which Q1 is an aromatic heterocyclic nucleus such that in the compounds of formulae Q1CH2R1 the methylene hydrogens are active hydrogens and R1 is a hydrogen atom or an aliphatic or cycloaliphatic group; and each X is a chlorine or bromine atom.
This invention also provides a squaric acid derivative of the formula: 
wherein Q1 is a heterocyclic nucleus such that in the compounds of formulae Q1CH2R1 the methylene hydrogens are active hydrogens and R1 is a hydrogen atom or an aliphatic or cycloaliphatic group; and A is a chlorine or bromine atom, a hydroxyl group or an esterified hydroxyl group.
Finally, this invention provides a process for the preparation of a squarylium compound of the formula: 
wherein Q1 and R1 are as defined above, and Q3 is an aromatic nucleus bearing an electron-donating group. This process comprises reacting a squaric acid derivative of Formula II as defined above, with a compound of the formula Q3H. This reaction will hereinafter be called the xe2x80x9caromatic dye-formingxe2x80x9d reaction of the invention.
It will be noted that the symbol Q1 has been used for both a divalent grouping in Formula I and a monovalent grouping in the formula Q1CH2R1. This apparent anomaly arises because the bond orders in the compounds of Formula I (and indeed in the compounds of Formulae II-VII also) are not integral. For example, the dye A shown in FIG. 1 is actually a resonance hybrid of the form shown and: 
(with contributions from other resonance forms). Thus, whether Q1 is drawn as divalent or monovalent depends solely upon which of the contributing resonance forms is drawn, and similarly for Q2. On the other hand, the compounds of formula Q1CH2R1, such as the salt B shown in FIG. 1, are not resonance hybrids to any significant extent, and thus in this formula Q1 is correctly shown as monovalent. The Q1/Q2 nomenclature employed will thus be clear to skilled chemists.
The dyes produced by the processes of the present invention may be cationic, anionic or non-ionic. When neither of the nuclei Q1 and Q2 (Q1 and Q3 in dyes of Formula VII) carries any charged substituents, the Q1Q2-squarate moiety (or the Q1Q3-squarate moiety; either moiety is hereinafter called simply the xe2x80x9cdye moietyxe2x80x9d) is uncharged, and hence the dye is non-ionic. However, if either of the nuclei Q1 and Q2 (or Q1 and Q3) carries a negatively or positively charged group (for example a xe2x80x94COOxe2x80x94 or trialkylammonium substituent), the dye will be anionic or cationic respectively, and will contain a counterion.
When such a counterion is present, it may be any counterion which is not incompatible with the dye moiety and which thus provides a stable salt. The choice of counterion may be important in ensuring the solubility of the dye in various media, and reducing or preventing aggregation of the dye; minimizing aggregation of the dye is highly desirable since such aggregation can significantly reduce the apparent extinction coefficient of the dye in polymeric media.
Similarly, if the nucleus Q1 or Q2 does not carry any charged substituents (such nuclei being generally preferred in the present processes), the xe2x80x9ccompoundsxe2x80x9d Q1CH2R1 and Q2CCH2R2 used in the present processes are cations. The counterion present may be any counterion which provides a stable salt and does not interfere with the relevant reactions. Typically, large fluorinated anions, such as trifluoromethane sulfonate and tetrafluoroborate have been found to give good results in the present processes. The nuclei Q1 and Q2 may, however, bear charged substituents and thus in some cases Q1CH2R1 and Q2CH2R2 may be neutral compounds which do not require the presence of a counterion.
It may often be found convenient, for synthetic reasons, to prepare a desired moiety with one counterion and thereafter to effect a counterion exchange to form a different salt of the same moiety. Methods for such counterion ion exchange are well known to those skilled in the art.